1. Field of the Invention
The present invention relates generally to a distance measuring device for cameras; and, more particularly, to a distance measuring device based on the so-called active method, in which a light receiving element receives reflections of beams of light emitted toward an object from a light emitting element, and the object distance is measured based on the conversion of the reflected beams of light into electrical signals by the light receiving element.
2. Description of the Prior Art
Conventionally, distance measurement in a camera involves a distance measurement system in which a beam of light is emitted toward an object from a light emitting element, such as a light emitting diode (LED); and the light reflected from the object is received as a spot of light by a light receiving element, such as a position sensitive device (PSD). The object distance is measured based on the position of the spot of the reflected light, applying the principles of trigonometric surveying.
However, in conventional distance measuring systems used in cameras, when a part of the light emitted from the light emitting element does not impinge upon the main object; but, instead, continues past the main object to the background (such a phenomenon is called "vignetting"), the location of the center point of the reflected light on the PSD moves out of place, resulting in an erroneous distance measurement and thus a blurred picture.
FIG. 1 illustrates the operation of the distance measurement function of an active auto-focus process based on the trigonometric survey method. As shown in FIG. 1, light emitted from a light source 3 is caused to converge into a beam by light projecting lens 4 which shares optical axis L1 with light source 3, and is projected onto an object 5. The light reflected from object 5 passes through a light receiving lens 2 having an optical axis L2 parallel to optical axis L1 of light projecting lens 4, and forms an image at position P on light receiving surface 10 of PSD 1 positioned at the focal length f of light receiving lens 2 such that its center O coincides with optical axis L2 and its surface 10 is perpendicular to optical axis L2.
When the reflected light strikes surface 10 of PSD 1, electric currents flow via two electrodes A and B positioned on opposite ends of PSD 1 according to the intensity of the reflected light. These currents are designated in FIG. 1 as ia and ib and are defined by the formulae EQU ia=Rb/(Ra+Rb)(IO) (1) EQU ib=Ra/(Ra+Rb)(IO) (2).
In the formulae, ia and ib are the electric currents which flow via the electrodes A and B, respectively, the electric current generated according to the intensity of the reflected light is IO and the resistances between point P and electrodes A and B are Ra and Rb, respectively. As also shown in FIG. 1, the distance between center O of surface 10 and point P is x and the effective resistance length of PSD 1 is m.
As is known in the art, the non-resistance distribution over the length m is uniform on the PSD 1. Therefore, the resistances Ra and Rb are proportional to the distances between point P and electrodes A and B, respectively, and can be expressed using distance x and length m. In other words, the above formulae (1) and (2) can be written as follows. EQU ia=(m/2+x)/(m)(IO) (3) EQU ib=(m/2-x)/(m)(IO) (4)
In the above formulae (3) and (4), since IO is an unknown which is proportional to the intensity of the reflected light which forms an image at point P, the unknown x cannot be obtained using only one of the above formulae (3) and (4). Therefore, in order to eliminate the influence of the above IO, the ratio K between ia and (ia+ib) is generally sought using the relation of IO=ia+ib. That is, ##EQU1## is obtained. As a result, information which contains only the unknown x indicating the position of point P can be obtained. In other words, by measuring ia and ib, x can be obtained.
If the distance between light projecting lens 4 and the object 5 is D, the focal length of light receiving lens 2 is f (usually the PSD is located in the vicinity of the focal point of the light receiving lens), the distance between optical axis L1 of light projecting lens 4 and optical axis L2 of light receiving lens 2 (i.e., the base-length) is BL and the object whose reflected light forms an image at center O of surface 10 of PSD 1 is an object at infinity; the following formula is obtained via the trigonometric survey method. EQU BL/D=x/f (6)
Therefore, D is obtained as EQU D=(BL)(f)/x (7)
Therefore, if the denominator x in the above formula (7) can be obtained by obtaining the above-mentioned ratio K, the distance D between light projecting lens 4 and object 5 can be calculated.
The vignetting phenomenon will now be explained. Specifically, in the above explanation, it was assumed that the position where the reflected light from the object forms an image was at point P. In truth, however, image formation takes place not only on point P but across surface 10 of PSD 1. Therefore, image formation point P should be deemed to be the location of the center point corresponding to the light acceptance intensity distribution of the reflected light.
If the light source's width of tip in the direction of the baseline is W, the focal length of the light projecting lens is f1 and the focal length of the light receiving lens is f2; the width of light source image WPSD formed on surface 10 of PSD 1 is expressed as EQU WPSD=(W)(f2/f1) (8)
As shown in FIG. 2, when the quantity of received light is symmetrical across the center line of WPSD, the location of the center point, namely image formation point P, is determined to be the center of WPSD.
When the vignetting phenomenon takes place, however, as shown in FIG. 3, reflected light is received on the PSD surface, as shown in FIG. 4. Incidentally, the same reference numbers as in FIG. 1 are used in FIG. 3 for corresponding elements. In FIG. 3, an image 3' of light source 3 is projected onto an object 5. The shaded area of the image refers to the portion which is correctly projected onto the object and the reflected light of this portion is received on the surface 10 of PSD 1. However, because the remaining portion of the image other than that indicated by the shaded area continues through to the background, the reflected light of the remaining portion is barely received on the surface 10. The location of the center point (the image formation point) in such a case is point P' (FIG. 4). The difference in the positions of the center points P (which does not entail the vignetting phenomenon) and P' manifests as an error in distance measurement due to the vignetting.
In addition, as more light continues through to the background in the baseline direction (right to left in FIG. 3), the error in distance measurement caused by vignetting becomes larger. Further, when the amount of reflected light is as shown in FIG. 4, peripheral rounding and tail are caused due to flaring of the light emitting and receiving lenses.
In the construction depicted in FIG. 3, i.e., when the baseline direction is from right to left when seen from above the camera (i.e., the distance measuring system), and the light emitting means is located on the left and the light receiving means is located on the right; if the projected light source image is off to the right of the object, the location of center point P' of the received light image on the surface 10 of PSD 1 provides information that the object distance is shorter than when the location of center point P is obtained with no vignetting. Such vignetting is called "close vignetting". Conversely, when the projected light source image is off to the left of the object, the location of center point P' of received light on the surface 10 of PSD 1 provides information that the object distance is longer than when the location of center point P is obtained with no vignetting. Such vignetting is called "far vignetting".
Further, in a converse construction in which the light emitting means is located on the right and the light receiving means is on the left, if the projected light source image is off to the right of the object, the location of center point P' of the received light image on the PSD 1 surface provides information that the object distance is longer than when the location of center point P is obtained with no vignetting; and if the light source image is off to the left of the object, the location of center point P' of the received light image on the PSD surface provides information that the object distance is shorter than when the location of center point P is obtained with no vignetting.
Among vignetting-corrective distance measurement systems proposed to date, the following two types of systems are known.
The first type of system is a distance measuring system in which two light receiving units are located over a certain baseline length at equal distances from and on either side of the light emitting device. Referring to FIG. 17, light source 3 is positioned between PSDs 1-L and 1-R. Light from the light source 3 is projected through light projecting lens 4 onto the object 5. The light receiving lenses 2-L and 2-R are positioned to focus the projected image 3' of light source 3 onto the surfaces of PSDs 1-L and 1-R, respectively. The shaded area in FIG. 17 indicates the portion which is correctly projected onto the object, and the remaining area of projected image 3' indicates the portion of the projected light which continues through to the background.
When a vignetting phenomenon as shown in FIG. 17 occurs (the light source image is off to the right of the object), the location of the center point of the light emitted from light source 3 and reflected by the object 5 differs from the correct location of the center point that would be obtained when there is no vignetting on PSD 1-L, indicating an object distance longer than the correct one (far vignetting). On PSD 1-R, the location of the center point of the light emitted from light source 3 and reflected by the object 5 differs from the correct location of the center point that is obtained when there is no vignetting, indicating a shorter object distance than the correct one (close vignetting). By calculating the object distances obtained on PSD 1-L and PSD 1-R (using averaging, etc), an accurate distance measurement output can be obtained.
A second type of conventional distance measuring system is explained width reference to FIG. 18. In FIG. 18, light sources LED1, LED2 AND LED3 project light through light projecting lens 4 toward object 5 to form projected images 3'-1, 3'-2 and 3'-3, respectively. The system of FIG. 18 is a multiple-point distance measuring system based on a method in which the distance measurement information from LED1, which is experiencing vignetting, is corrected by distance measurement information from another LED (LED2 or LED3) in its vicinity.
If it is assumed that the vignetting phenomenon shown in FIG. 18 occurs, the information from LED1 indicates a distance measurement shorter than the correct one (close vignetting), while the information from LED2 is correct because the light emitted by it is accurately projected onto the object 5. Because the light from LED3 passes through completely to the background, its information will indicate a distance measurement much longer than the correct object distance.
In the case of a multiple-point distance measuring system as above, it is common to use the information indicating the shortest object distance from among the information given from LED1, LED2 and LED3. Therefore, if vignetting correction is not performed under the situation shown in FIG. 18, the object distance information from LED1 is used, resulting in an erroneous distance measurement. However, the error in-distance measurement is corrected in this system using the following method.
In this system, the reflectance of the object is assumed to be constant and it is presumed that the reflected light from the LED which gives the shortest object distance information constitutes the greatest amount of reflected light. Thus, in FIG. 18, the amount of reflected light is the largest from LED2, followed by LED1 and then LED3. As a result, it is understood that the amount of reflected light from LED1, which gives the shortest object distance information, is not the largest. Therefore, it is determined that this is because LED1 is experiencing close vignetting, and the object distance information for LED1 is not adopted. Since the information for LED2, which gives the next shortest object distance information, is adopted, a correct distance measurement output can be obtained.
In the first type of system described above, because two light receiving devices are necessary, the system requires one more set of a light receiving lens and light receiving element than in a conventional distance measuring system, which leads to a higher cost. In addition, because the light receiving devices are located symmetrically across a light emitting device over a certain baseline length, it requires twice as much space as a conventional system, which leads to a larger camera size.
In the second type of system described above, because the reflectance of the object is assumed to be uniform, if the reflectance of the object is in actuality, different for each LED, it quickly becomes unclear whether or not there is any vignetting. In addition, because a comparison with a nearby LED is necessary, this system can be applied only for multiple-point distance measurement and this method cannot be used for single-point distance measurement.